1. Field of the Invention
The present invention relates to a method and apparatus for detecting cable faults in a network such as a fieldbus network.
2. Description of the Related Art
A fieldbus network is a two-wire network capable of delivering DC power and bidirectional digital communications to remotely mounted field devices. The digital communications are typically carried on a 31.25 kHz carrier signal in the standard defined fieldbus network. Fieldbus networks are advantageously used in industrial control systems and particularly in control systems that provide distributed control incorporating field devices having local processors. Various field devices might be attached to be powered from and communicate over the network, including controllers, actuators and sensors. Multiple field devices can be attached to one fieldbus segment. The fieldbus is typically implemented on the physical level as two wire shielded cable. More details can be found in the International Electrotechnical Commission standard IEC 61158-2 which is specifically for industrial networks and promoted by Foundation Fieldbus and PROFIBUS organizations.
Fieldbus power supplies differ from conventional power supplies in that fieldbus power supplies include extra components to keep fieldbus communication signals from being fed into and interfering with the power supply. Thus, fieldbus power supplies include conditioning circuitry, often in the form of series inductors, to isolate the regulated DC power supply from the AC component used for communication across the network.
According to the IEC 61158-2 standard, a fieldbus segment can have an overall cable length of up to 1900 meters. The standard states that each fieldbus segment can power up to a maximum of thirty-two field devices and those devices are attached in parallel across the two-wire fieldbus network. The input impedance of any field device according to the standard is no less than 3 kΩ. Therefore, the total equivalent input impedance of field devices per segment can be below 100Ω. On the other hand, the typical loop resistance of a type A fieldbus cable, such as those made by Kerpenwerk GmbH, is 45 Ω/kM for Foundation Fieldbus or PA applications and 110 Ω/kM for DP PROFIBUS applications. Therefore, the cable loop resistance can be as high as 100Ω or 220Ω for the approximately two kilometer allowable segment length for networks configured according to the standard.
If a short circuit occurs at the far end of a standard fieldbus network cable, the conditioned power supply will continue feeding current to the load, because the cable loop resistance seen by the conditioned power supply is approximately equivalent to a fully loaded fieldbus segment. Under these circumstances, the power supply cannot differentiate between a true load and a cable fault on the basis of the current drawn, because the current drawn does not exceed the threshold value set by the power supply specification. For instance, a 24 V DC conditioned power supply can deliver up to 350 mA as maximum current per segment. If there are eight field devices attached to a fieldbus segment having a length 1500 meters, the cable loop resistance will be 75Ω. Each field device draws 20 mA, so the total current drawn from the system is 160 mA. Should a cable fault occur at the far end of the cable, the drawn current will be 24 V÷75Ω=320 mA, which is below the threshold value. Therefore, the power supply reports no failure.
Conditioned power supplies, whether isolated or nonisolated, are typically equipped with electronic short circuit protection. These circuits are not designed to detect cable faults over a long cable. For a short cable, the loop resistance allows the power supply to output current above the threshold value. The protective circuitry built-in inside the power supply will limit the drawn current. Other types of power supplies will continue to output current at full capacity. If a conditioned power supply fails to detect the cable fault, it can corrupt the communication signals on the fieldbus network. It may also cause damage to the power supply modules because of the excessive heat generated as a result of high inrush current or spiky noise switching signals generated by the DC/DC converter are fed back into the system via the DC bus.
U.S. Pat. No. 6,912,671 describes wiring fault detection that measures and diagnoses transmitted protocol signals over a fieldbus network. This measurement is performed at the controller side, that is, wiring connects the controller to the conditioned power supply and then the cable of the fieldbus network is connected from the conditioned power supply to the field devices. In this configuration the controller makes its measurements through the conditioned power supply. In intrinsically safe applications, the wiring that carries transmitted signals is physically isolated from the field side which powers the field devices. If a cable short circuit occurs on the field side in such an intrinsically safe application, the '671 patent technique cannot detect the fault on the field side because it is isolated from the fieldbus network by the front-end transformer of the conditioned power supply. Because of its configuration, the '671 patent technique can detect a cable short circuit between the controller and the conditioned power supply, but not in the fieldbus network cable. In addition, the '671 patent technique is not compatible with detecting a cable fault when communication signals are present on the fieldbus network.
When a cable fault occurs in these conventional configurations, network signal integrity may be lost but the conditioned power supply electronics continue to indicate that no cable fault has occurred. No alarm is reported or sent to alert the user or the host. Such faults are difficult to resolve and to locate within a network. Moreover, the nature of the fault may be misunderstood, leading to further delay. Here, the cable is faulty but because the power supply cannot detect this type of fault, the power supply may erroneously be considered faulty and need to be verified, which is undesirable.